Implantable medical devices such as cardioverter/defibrillators are commonly configured to treat cardiac arrhythmias by delivering high voltage energy pulses to cardiac tissue. Implantable defibrillators commonly delivery therapy by way of electrodes positioned within or near the heart of the patient. Such therapy includes defibrillation therapy, which utilizes a sudden, high energy pulse designed to shock the heart of the patient out of a cardiac arrhythmia if and when a cardiac arrhythmia occurs. Implantable defibrillators also commonly incorporate pacing therapy, which utilizes very low energy pulses designed to trigger cardiac contractions in lieu of an adequately frequent natural heart beat of the patient.
Implantable defibrillators commonly incorporate a power source, such as a battery, which provides operational power to the componentry of the defibrillator, including electronics which manage the function of the device, monitor the condition of the patient in which the device is implanted and deliver therapy to the patient. Many or most device functions operate effectively continually, such as sensing the cardiac condition of the patient, or frequently, such as cardiac pacing therapy delivery in certain patients, and thus account for steady, predictable and, usually, low-level drains on the battery capacity. Defibrillation therapy, by contrast, usually occurs very infrequently in most patients, commonly with months or years between defibrillation therapy deliveries, owing to the generally infrequent occurrence of arrhythmias which require treatment. As such, defibrillation therapy is, from a standpoint of battery management, a large, sudden, essentially random drain on the battery of the implantable defibrillator.
Because implantable defibrillators often provide life-sustaining therapy to the patients in which they are implanted, it may be essential to the well-being of the patient to understand how long the battery may be expected to last until the battery will be discharged to a point of being unable to provide reliable therapy. It is known in the art that the terminal voltage of batteries often utilized in implantable defibrillators typically corresponds, to some degree, to the remaining charge in the battery. For instance, as the remaining charge decreases, the terminal voltage likewise decreases. However, the terminal voltage may not, and often does not, correspond to the remaining charge in the battery in a wholly straight-forward and predictable relationship.